Battery Chargers

[tuXguy15]

Stinglikebee

1. 6 Batteries
2. Yes
3. Yes
4. Yes
5. Yes
6. Yes
7. No

[tuXguy15]

Adciv
They aren't reaching 15v like we want. It goes to 13 for a charge and the second we take them off they drop to 12.6-12.8v and after 1 match they are below 12v. When we get low on voltage we drop to around 6-7v when the compressor runs.

[tuXguy15]

Here is a picture that we drew out in paint to try to demonstrate how the batteries are supposed to be charged for maximum performance.

[adciv]

I'd like to know where your charge curve came from. It is very different from any I've seen before. The typical one I've seen is on the link below (scroll down for lead-acid).
http://www.maximintegrated.com/app-n...dex.mvp/id/680

[fb39ca4]

Originally Posted by mman1506

I have that charger but it only changers at 50w vs 150w on the other one(Meaning on a 12v battery it will charge up to 4.1 amps on the 50w or 12.5 amps on the 150w) A boat battery makes a great portable power source.

It charges fine when I set it to 12v/6A. It just gets a little warm, that's all.

[mman1506]

Originally Posted by fb39ca4

It charges fine when I set it to 12v/6A. It just gets a little warm, that's all.

did you check the actual amperage on the screen. You can input up to 6A but it will only go up to 50 watts as it is the lower limit.

[mman1506]

Originally Posted by tuXguy15

Here is a picture that we drew out in paint to try to demonstrate how the batteries are supposed to be charged for maximum performance.

Again your charge graph is wrong. The higher the charge current the higher the battery voltage. if you want to put the most amount of power in while staying at the same voltage you lower the charge current which lowers the battery voltage. This allows you to put more power in while staying below cut off voltage.

To test this check your battery voltage while it is charging at 6A then check it again right after unplugging it from the charger. The voltage will drop proportionally to the charge current.

[Tom Line]

Originally Posted by tuXguy15

Adciv
They aren't reaching 15v like we want. It goes to 13 for a charge and the second we take them off they drop to 12.6-12.8v and after 1 match they are below 12v. When we get low on voltage we drop to around 6-7v when the compressor runs.

This is absolutely standard behavior for the batteries we use (and pretty much all 12 volt lead acid batteries).

All batteries all drop below 12 after a full match. If you are running a compressor, it will draw down the available voltage, as will any load placed on the battery. Batteries will drop when a heavy load is placed on them. With a completely charged battery, running 4 cims at stall or near it (say under full acceleration) will drop a fully charged battery down under 10 volts temporarily. Once that load is removed, it will return back up near it's previous point.

To me, it sounds like you're expecting some type of different behavior out of the battery, but everything I've read so far suggests these batteries are just fine.

Could you further clarify what you mean when you say your batteries have gone bad? Using them in the manner you are describing: charging to a surface charge of 13 (true charge of 12.6-12.8) then running a match, then recharging should absolutely not hurt your battery at all.

Indeed, if you read the specifications for the batteries we use (available at AndyMark to save you time) you'll see that the maximum charging voltage is 15 volts. However, once your battery is fully charged (amperage going into the battery is no longer increasing), your charger will drop to a floating voltage charge of 13.5 to 13.8 volts to keep the battery topped up.

These batteries will not read measure 15 volts acrossed the terminals when you are done charging them.

[Michael Hill]

Originally Posted by Al Skierkiewicz

Guys,
There are a few things about batteries you need to know. A battery might show something above 12 volts (no load) when it comes off the charger due to a condition known as 'surface charge'. This voltage will drain back down to the 12 volts as soon as you start to draw current from the battery. That is why no load battery testing is inconclusive when checking the state of charge or the condition of the battery. Charging SLA/AGM cells requires that a voltage higher than the cell voltage be forced on the battery. The higher the voltage, the more charge current is pushed into the battery to reverse the chemical reaction that takes place inside. The manufacturers state that currents in excess of six amps will damage the battery. However, the smart chargers provided in the KOP and equivalents will back down from the six amp maximum as needed during the charge cycle. So setting the charger at 6 amps does not mean it will provide six amps during the entire charge cycle. Any additional current in excess of what can be used to reverse the chemical reaction will generate heat. In our batteries this excess heat can cause boiling of the electrolyte within the cell, deformation of cell plates, leaking of the battery and explosion in rare cases. That is why inspectors will ask you to remove battery chargers that are in excess of six amps and are not smart chargers. We regularly will find teams trying to use 50 amp car chargers in the pits. These are extremely dangerous for use in the pit and will guarantee early battery failure with no warning. While the KOP charger does allow current selection below 6 amps (i.e.4 and 2 amps) selecting these lower settings will not measurably extend the life of the battery but if you have the time to wait, the lower current may be a more efficient charge method.
The best method of checking battery state of charge is under a controlled load. The Battery Beak is an excellent simple charge tester that you can carry in your pocket. For really accurate checks that mimic the manufacturer's specification, use a West Mountain Radio CBAIII.

I was seriously hoping Big Al would come in this thread.

[Al Skierkiewicz]

Guys,
I am afraid you are confusing things a little because of the common terms. In SLA/AGM batteries, in order to force charge current to flow, the external power supply must have a supply voltage higher than the "cell voltage". So when you are measuring a battery that is connected to a charger and the charger is supplying current, you are measuring the supply voltage not the battery voltage. You must remember that the battery has an internal resistance and that you are measuring the battery cell voltage plus the voltage across the resistance. The voltage drop across the internal resistance will increase as the charge current goes up. However, there are limits on the conversion of that current into reversing the chemical reaction within the battery. No matter how much current you attempt to force into the battery, that current which is in excess of what can be used for charging is lost as heat in the internal resistance of the battery. Once fully charged, all excess current is converted to heat.
So when charging at 2 amps, the supply voltage is set by the charger to produce 2 amps of charge current, to start. This will fully charge the battery, it will just take longer and produce less heat. To pick an arbitrary value, say you measure 12.8 volts on the power supply at 2 amps. At 6 amps, that voltage will increase to perhaps 14.5 volts, the battery will still fully charge, but in less time than the 2 amp setting. However, the increased current will also generate increased heat in the internal resistance. The internal resistance is 0.011 ohms. It is a simple matter to make a calculation. At 2 amps, 0.044 watts will be dissipated in the internal resistance. At 6 amps close to 0.4 watts will be dissipated. The remainder will be used in the charge chemical reaction (which also produces some heat). If we used a constant current charger then at the time that the chemical reaction is finished, all power will be dissipated by the internal resistance of the battery or nearly 72 watts. Thankfully, SLA chargers do not use a constant current charge method and most smart chargers never will charge at the rated current for their entire charge cycle. Please keep in mind that different battery chemistry have different cell voltages. Our batteries are very close to 2 volt/cell. Your car battery is more like 2.3 volts per cell so it will show 13.8 volts when fully charged. Alkalines are 1.5 volts per cell, NiCad is 1.2 and lithium is 3 volts per cell.
As I explained before there is always the "surface charge" that will confuse your readings if you remove the battery from charger and then immediately measure the terminal voltage. It is likely to measure as much as 14 or 15 volts but it is a lie. Leave the battery for a while or connect it to a load and the terminal voltage will fall back to 12 volts. Make that an excessive load like six motor drives and the terminal voltage will fall well below 12 volts, again because of the voltage drop across the internal resistance. The Battery Beak switches in different loads to make a quick calculation for terminal voltage, capacity, and internal resistance. The CBAIII simply charts the terminal voltage vs. time while connected to a dynamic load. The resulting chart will mimic the discharge graphs supplied by the manufacturers and will make Amp Hour calaculation based on the data.

[Michael Hill]

Al, what is the ideal charge rate for these batteries? The data sheet for the ES17-12 says no more than 5.4 amps. How much of a risk are we at by using the 6A initial charge rate? I've read somewhere that the issue of gas buildup really only happens when you overcharge, and that higher charge rates shouldn't hurt it much at all (even up to C/2) provided you don't overcharge. I ask this because I've been tossing the idea around of building a (ridiculously overcomplex) battery cart using my own charging circuit (using a lead-acid battery management IC of course...I'm not going to screw around with the charging profiles). However, I do have to tell it the max charge rate. What are your thoughts?

[Al Skierkiewicz]

Mike,
The manufacturer's spec is based on constant voltage dumb charging, so 5.4 amps is something they are comfortable with. Since the KOP charger is a smart device it uses typical lead acid techniques like delta V or some other profile. This where the charger is pumping current periodically and then testing the the battery without charge current. It compares the level of the battery now compared to previous samples and adjust the current (voltage) accordingly. The only improvement on this method would be to monitor internal temperature as part of the charge cycle. As the battery nears and reaches full charge the delta V levels off and actually starts to descend at full charge. At this point most chargers will stop charging or move into a maintenance charge cycle that is similar to trickle charging.
Something to keep in mind is that dumb chargers are usually unfiltered, rectified line voltage so the current meter is actually only reading average current but the peaks are much higher. Although the manufacturer that I spoke with a few years back was reluctant to agree, it is my thought that excessive charging currents (which require higher voltage to produce) may actually cause arcing between the closely spaced plates in AGM cells. If this is actually occurring, then the damaged plates can have reduced area and therefore reduced capacity or in rare cases, the plates may short internally. We know that some teams have experienced shorted cells, but in many cases that I have examined, the case showed signs of rough handling. In some cases there was no external damage visible so I have to conclude either a defect in manufacture or an arc internal to the cell. In a few of these cases, a charger defect was discovered. As you investigate charging circuits you will discover that all of them have some power device that feeds the charge current to the battery. Often these are similar devices to the MOSFETs you would find in a Victor or Jaguar. If that device should short on failure, you would deliver full power supply voltage to the battery under charge. I can't tell you that this was the case in the battery failures that have been reported simply because there are so few failures.
As to gas generation, high charge currents produce heat in the electrolyte in the glass mat. If this temperature is high enough, the electrolyte will start to boil. Also due to the electrical action some of the water will actually disassemble into separate hydrogen and oxygen atoms. Combine high temps, boiling electrolyte and hydrogen formation and the internal pressure of the battery will cause some venting through the vent plugs in each cell. In our batteries this is nothing to worry about as the gas production is relatively small and confined to the battery in most cases. As charging tapers off, the gases recombine and are reabsorbed by the glass mat.

[tuXguy15]

I am trying to say that it is safe and good to do a 6 Amp charge but not to start at directly. You want to start at 2 amp and escalate your charge to 6 amps because doing a constant 6 amp charge damages the batteries. My team along with other teams have tested this and the batteries that go from 2 to 4 to 6 amps last a lot longer than the constant 6 amp batteries. The voltage is also higher on the 2-6 amp charged batteries. I'm just asking for ideas that we can use to get a charger or build on that you can switch from 2 to 4 to 6 amps when you want.

[Al Skierkiewicz]

Devin,
There are so many variables in battery life. The greatest reduction in our battery use comes not from over charging but the high demands of discharge during a match. While our batteries are capable of extreme currents (600+ fully charged) they are really designed for discharge life of 1/10 C or about 2 amps. I have not read any articles that relate the stepped charge you are proposing to extended battery life. It won't hurt, I just don't think it will help that much value. You might consider reverse engineering the KOP charger to see what they do for the different charge rates and come up with a switch to perform that function. The reason the KOP charger has the different positions is to allow charging batteries of lower capacity than the KOP batteries

[tuXguy15]

Ok we will start putting together ideas and we dont have a huge load on our batteries during the match.